Russian professor is using a replacement for the ruler.  In this case it's a chair

 

This is an example of "Русская смекалка"

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Wouldn't a book have been a much simpler choice?

Or <gasp> drawing the line by hand?  

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это смотря какой угол нужен..для следущего уравнения, может, еще и стол пригодится

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A mozhet on novuyu formulu vivel: sport + matematika = dolgoletie?!

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"Some say the Muses are nine: how careless! Look, there's Sappho too, from Lesbos, the tenth." Plato